The present invention relates to a process for catalytic decomposition of nitrogen protoxide (N2O) to nitrogen and oxygen and to its use for removing protoxide from gas mixtures which contain it, in particular for removal from the emissions of nitric acid and adipic acid plants.
Nitrogen protoxide is a harmful greenhouse gas, much more powerful 1 than carbon dioxide; moreover, it takes part in the stratosphere in the reactions which lead to destruction of the ozone layer.
The main industrial sources of the generation of nitrogen protoxide are plants for producing nitric acid and adipic acid (a monomer used in the preparation of nylon 6,6 and 6,12).
Nitrogen protoxide is present in the emissions from adipic acid plants in considerable amounts: a typical composition comprises, in percentage by volume: 30% N2O, 2% CO2, 2.5% H2O, 8-12% O2, 50-150 ppm NOx.
The emissions of nitric acid plants generally contain 300-1700 ppm N2O, 100-2000 ppm NOx, 1-4% O2, the rest being nitrogen.
The emission of N2O from nitric acid and adipic acid plants is predicted to grow by approximately 16% over the period 2005-2020.
Several catalysts are known which are used to decompose N2O. The main ones are constituted by noble metals supported on metallic oxides of different kinds, zeolites substituted with transition metal ions or on which metallic oxides and anionic clays are supported, such as for example hydrotalcites constituted by mixed hydroxides with a stratified structure in which exchangeable or non-exchangeable anions of different kinds and water molecules are inserted between two layers.
All these catalysts have the drawback of not being thermally stable: the noble metals supported on metallic oxides because at high temperatures
the particles of the metal tend to sinter, with consequent deactivation of the catalyst; the clays and the zeolites because their structure tends to collapse and thus loses its initial catalytic properties.
Catalysts are known (U.S. Pat. No. 5,705,136) which are constituted by oxides such as MnO, CuO, NiO and CoO supported on MgO, CaO, ZnO TiO2, Al2O3—ZnO, Al2O3—TiO2, and the like. Preferably, the catalysts contain CoO supported on MgO.
N2O conversions are high.
Structures such as hydrotalcite, such as for example Cu3Mg5Al2(OH)20CO3 3H2O, Mn3 Mg5Al2(OH)20CO3H2O, can also be used.
It has now been found unexpectedly that the catalysts specified hereafter have a high catalytic activity in the decomposition of N2O to nitrogen and oxygen and a satisfactory thermal stability, and are able to keep their activity unchanged for long periods of time.
The catalysts comprise mixed oxides of copper, manganese and rare earth metals, which are present in the following composition, expressed in percentage by weight of CuO, MnO and oxide of rare earth metals in which the metal is present in the lowest valency state: 50-60% MnO, 20-45% CuO, 5-20% rare earth metal oxide.
Further the invention comprises a process for removing nitrogen protoxide from gas emission from adipic acid plants comprising contacting with a catalyst which comprises mixed oxides of copper, manganese and lanthanum having a composition expressed as percentage by weight of 55-60% MnO, 25-40% CuO, 9-14% La2O3.” In this process, the gas emissions released by the adipic acid plants may be made to pass over a fixed catalyst bed at temperatures from 600° C. to 700° C.”